Listening #29

Nothing is wonderful when you get used to it.—E.L. Howe

Before abandoning the subject of turntable power supplies for at least one whole month—that's a solemn promise—I'm here to take another crack at it. And this time I'll keep in mind that not everybody owns a Linn Sondek LP12.

But I'll bet my Social Security fund that most Stereophile readers who play LPs at home do so on a belt-drive turntable that has an AC motor. To those readers I say: I want you to get all the help you need.

There's nothing wrong with listening to vinyl, of course. It's simply that vinyl lovers are in constant need of assistance, if only because LP playback is subject to a nearly endless variety of obstacles, and so would seem capable of a similarly endless degree of refinement: The music on an LP exists as a physical structure whose details are almost unthinkably small and impossible to measure in any meaningful way, apart from listening. And unlike all digital storage media, whose contents are finite and whose playback refinements are in fact limited, the contents of an analog storage device can never be fully known. So the admittedly destructible vinyl record has something of the infinite about it—certainly more so than the CD, even if the latter were indestructible. Which it isn't.

The resonant frequency of the beast
So let's have another look at AC motors and how noisy they are—although that isn't entirely a bad thing, given that all loudspeakers are themselves AC motors, and the noise they make is literally music to our ears.

In most other ways, though, considering the record player's role as a sort of a hypersensitive seismograph—its job is to measure the amplitude of groove modulation against the time constant of the rotating platter, and those changes in amplitude can be smaller than a half wavelength of light—the need to eliminate every last trace of noise and vibration is obvious. And right there, just inches from the stylus, is the most abundant source of noise in the room: the motor. That damn AC motor.

Now: The speed of an AC motor is determined by the AC power-supply frequency. And the motor used in most turntables from Linn, Rega, Roksan, Nottingham Analogue, and others is a 24-pole type. Using the formula N1 = 120 x f / P, where N1 is the rotor's speed in rpm, f is the power-supply frequency, and P is the number of poles, we can see that, in Europe, where the AC line frequency is 50Hz, the motor will turn at 250rpm. (We might have just looked at the motor casing, which has the words 250rpm on it.) But in the US and Canada, where the AC line frequency is 60Hz, the same motor turns faster, at 300rpm.

Divide 300rpm by 60, the number of seconds in one minute, and the answer you'll get is 5—which describes the frequency of the motor noise in Hz. Look how that compares with the devilishly low 4.1666Hz it exhibits in the UK! (footnote 1)

And 5Hz is just the fundamental. If any one coil is loose or otherwise different from any of the others, the motor will also vibrate at a frequency equal to the total number of poles (which is, again, 24 in our case) times the revolutions per second (which we've already decided is 5), with sidebands at every multiple of the rotating frequency. Also, any rotor or stator eccentricity—which is virtually inevitable—will create a shifting air gap, resulting in a vibration at a frequency greater than twice the line frequency (again, 60Hz here in the Homeland) but less than the nearest rotating speed harmonic.

To put all of this into perspective: AC motor noise will affect the musical performance of any turntable, especially one that's used in the US or Canada and driven directly from the local AC. Even the fundamental of that noise is high enough in frequency that a turntable's suspension system, if any, can't be counted on to eliminate it, nor can the filter formed by the compliance of the drive belt and the rotating platter mass itself do a whole lot of good in that regard. And much of the sonic impact of that noise will be felt in the upper bass through lower midrange, where serious listeners are especially sensitive, and which carries a great deal of the rhythmic information in upbeat music.

Have a nice day!

Good speed hunting
What's a mother to do? Some people say that DC is the answer.

A permanent-magnet DC motor has many things going for it. A DC motor can be driven so that full torque is available at all speeds, yet it will also exhibit less noise and vibration than a comparable AC motor. And speed adjustability/variability is a lot easier to engineer into a DC than an AC motor system.

On the down side: All other things being equal, a good-quality DC motor is more expensive than its AC counterpart. It's also more subject to wear and tear.

Then there's the little matter of cogging—a steady oscillation peculiar to the DC breed. You probably observed a coarse version of it in your fifth-grade science class, when the simple electric motor you made out of nails and copper wire tended to stall every half-revolution. Cogging is less a problem today than it used to be, thanks to such refinements as high-flux neodymium magnets and skewed rotor structures, but turntable designers still have to take it into consideration.

And I haven't even used the S-word—as in servo. Most audiophiles over a certain age have owned at least one DC-drive turntable whose motor was topped with a little tachometer, the output of which was a control signal for a power-supply servo. That was done to keep the platter speed steady, and to avoid hunting—which the dictionary describes as "to oscillate alternately to each side (as of a neutral point) or to run alternately faster and slower."

But some people say that servos in turntables do more harm than good—mostly because servos have to wait for the system to make a mistake before (over)correcting it. Whether or not that's so, a negative perception of servo units in turntables seems to have kept a lot of designers away from DC. It even caused one manufacturer—Pink Triangle Products, who pioneered servo-controlled DC drive as well as other turntable innovations—to drop DC in favor of AC drive for a while, in response to dealer pressure. But then they went back to DC. But then they went out of business (footnote 2).

Then along came Origin Live, a small English company that got their start selling Rega tonearm modifications (footnote 3). Back in the 1990s, Origin Live offered a mod kit to owners of the Linn LP12 and other AC-drive turntables, which consisted of a DC motor and complementary power supply. Early versions of the kit used a 12V battery and charger, but the battery gave way to a much simpler rectification-and-regulation circuit that the user could mount inside the turntable or assemble into the project box of his or her choice.

Today Origin Live offers three levels of their DC drive accessory: The drive circuitry of the Standard version ($316) appears little changed from that of its simple predecessor, which I reviewed in the July/August 2000 issue of Listener. The Advanced ($540) and Ultra ($899) versions are more complex, and incorporate a proprietary load-sensing circuit said to maintain correct speed to within 0.1% in the face of varying amounts of groove drag. With all three versions the buyer gets: a 12V DC motor with a precision-bored pulley and a laser-etched, stainless-steel mounting plate; a nicely finished metal enclosure containing the control switch and all the motor-drive circuitry; an outboard transformer; a strobe disc for adjusting speed; and various fasteners and other bits.

No substitute for a handshake
Early this year I sampled Origin Live's Advanced and Ultra kits, and right off the bat I was impressed by how far the firm has come in terms of build quality and finish. The motor plate in particular has gone from Homo erectus to Homo sapiens sapiens in just a few years, and in terms of being accurately made and mounted, the alloy motor pulley—which wasn't bad to begin with—has no betters that I've seen. The drive unit and switch box are now built into a ruggedly handsome extruded enclosure finished in black or silver, depending on the product level, and the feel of the switch puts the early version to shame. There's even a neat little stainless-steel escutcheon, sized just right for the switch opening on an LP12's top plate.

Speaking of which: My last power-supply test wasn't too terribly long ago, and consequently I wasn't too terribly anxious to bring the Linn setup jig back out of the basement. I delayed the inevitable by first trying the Origin Live kits on my Rega Planar 9 turntable, which uses more or less the same Airpax AC motor as the LP12, and which is a good deal easier to work on: Removing the motor from a Rega takes all of five minutes, including the time it takes to disconnect the player from the hi-fi. The whole job, from needle drop to needle drop, was done in a half hour, with no soldering and no unpleasant bending.

An interesting aside: Origin Live's motor pulley has a slim, convex profile with distinct upper and lower ridges, and the company says it can be used with either a flat belt à la Linn and Roksan, or a thin, round-cross-section belt à la Rega. In the latter case they suggest that the belt can be aligned to ride against either the upper or lower ridge—left to its own devices, the belt will certainly migrate to one or the other. I took advantage of the situation by using two belts, as Rega intends the user to do with their stock player, coaxing one to the bottom of the pulley and the other to the top, where they stayed without complaint. There's no reason the same "upgrade" couldn't be performed with the less expensive Rega P3 or other similar players. (If you buy an OL motor kit for a humbler Rega and you want to try running two belts, your favorite Rega dealer can sell you an extra one for $24.)



Footnote 1: Not only that, but an AC motor, which has a notoriously nonlinear relationship between speed and torque, will behave differently in the US and the UK in terms of its response to groove drag—which behavior, like its different noise spectrum, cannot be compensated for by merely installing a smaller drive pulley. The evidence continues to pile up that certain British record players don't sound the same here as they do there, and that the differences tend to favor European record lovers. (As I explained in this space in the February 2005 issue, Americans have never heard the combination of a Linn Sondek LP12 and a Naim Armageddon power supply at its best.)

Footnote 2: Rumor has it that Pink Triangle is on its way back into the turntable business—although it won't be called Pink Triangle anymore.

Footnote 3: Origin Live, Unit 5 362B Spring Road, Sholing, Southampton, UK, SO19 2PB Tel/Fax: +44 (0)2380-578877. Web: www.originlive.com.

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